Vehicle navigation systems exist in which data concerning road segments in a geographical area is stored in a memory device as a road map data base. A navigation computer then calculates a desired route between a current specified position of a vehicle and a desired destination. This route comprises an ordered connected plurality of road segments selected from the road segments in the road map data base. These vehicle navigation systems then provide guidance instructions to the vehicle user to enable the vehicle to traverse the desired route by implementing maneuvers at the junctions, or nodes or intersections, at which the various road segments are connected. Typically, the route is calculated starting from the vehicle's current position.
In navigation systems such as those described above, sometimes it may not be possible or desirable to follow the guidance instructions for traversing the desired route. Sometimes, there is a road blockage which prevents travel on a specified road segment. Other times there are errors in the road map data base which result in the guidance instructions requiring the vehicle to turn onto nonexistent roads or to turn the wrong way down a one-way street. What typically happens in such situations is that the vehicle will go off route and some sort of route recovery is implemented.
In one prior system, such as the Philips "Carin" system, when the vehicle goes off route the system will automatically calculate a new route to the desired destination. This type of recovery is perfectly satisfactory if the reason the vehicle went off route was simply because the vehicle driver failed to implement a desired maneuver and there was no permanent restriction preventing the implementing of such a maneuver. However, if the reason that the vehicle went off route was because the guidance instruction would have instructed the vehicle to turn the wrong way down a one-way street or turn onto a road segment which had a permanent or relatively permanent blockage of traffic, it is quite likely that such prior navigation systems will merely reroute the vehicle such that the vehicle driver will again be instructed to implement the same undesirable maneuver. In other words, the prior automatic reroute navigation systems have no way of determining if the reason that a desired maneuver was not implemented was because of some actual undesirability of implementing the maneuver or if the maneuver was just not implemented because of a very transient type situation, such as the momentary inattentiveness of the driver or the fact that the driver happened to be in the wrong lane for making a desired turn maneuver. The end result is that prior automatic reroute systems may continually route the vehicle back to the same maneuver or obstruction.
Some prior navigation systems allow a vehicle operator to implement a new route to the same destination while the operator specifies the exclusion of particular road segments. This essentially corresponds to calculating an entire new route where the vehicle driver has to specifically identify by name or by location roads or road segments that are to be avoided and not made part of the new route. While many such prior navigation systems exist, the requirement that the vehicle operator specifically identify what road segments are to be excluded from a new route means that the route planning process is now substantially longer and more complicated. When a vehicle driver is already driving towards a desired destination on an initial route, requiring the vehicle operator to plan an entire new route by inputing to the navigation computer specifics as to what roads or road segments are to be avoided in a new route is not desirable. If the driver does not exclude some road segment or maneuver from the new route to be calculated, the new route may include the same problem that existed in the original route.
In prior vehicle navigation systems, routes are calculated based on the vehicle's current position and the position of the desired destination. However, if the vehicle is moving, an optimum route might not be selected because when the route is actually provided by the navigation computer at a subsequent time the vehicle's then current position may not correspond to the vehicle location for which the optimum route has been calculated. Thus, for example, a route may be provided which requires a turn onto a road segment which the vehicle may have just passed because the vehicle was moving at a substantial rate of speed when a command for a new route was generated and because the navigation computer required an appreciable time to calculate the new route. Thus while a new route has been provided, implementing this new route may be difficult if the vehicle was moving and an appreciable time is required for the computer to generate the new route. Current navigation computers do require a substantial calculation time due to the large number of road segments which may be involved in plotting a route. This type of deficiency apparently has not been considered by prior navigation systems.